android分析之Condition

Condition的含义是条件变量，其实现依赖于系统，一般都要配合Mutex使用，使用步骤为：给mutex上锁（Lock），调用wait等待“条件”发生，如果没有发生则re-wait（），最后释放mutex（unlock），并继续执行。所有等待（wait)同一个“条件变量(condition)”的线程都要使用相同的一把锁——这样相当于互斥操作该Condition。

// ---------------------------------------------------------------------------
namespace android {
// ---------------------------------------------------------------------------

/*
 * Condition variable class.  The implementation is system-dependent.
 *
 * Condition variables are paired up with mutexes.  Lock the mutex,
 * call wait(), then either re-wait() if things aren't quite what you want,
 * or unlock the mutex and continue.  All threads calling wait() must
 * use the same mutex for a given Condition.
 */
class Condition {
public:
    enum {
        PRIVATE = 0,//指定释放跨进程共享
        SHARED = 1
    };

    Condition();
    Condition(int type);
    ~Condition();
    // Wait on the condition variable.  Lock the mutex before calling.
    status_t wait(Mutex& mutex);//通过Mutex来对“条件变量”实现互斥访问——多线程、多进程场景
    // same with relative timeout
    status_t waitRelative(Mutex& mutex, nsecs_t reltime);
    // Signal the condition variable, allowing one thread to continue.
    void signal();
    // Signal the condition variable, allowing all threads to continue.
    void broadcast();

private:
#if defined(HAVE_PTHREADS)
    pthread_cond_t mCond;//Linux的pthread_cond_t类型
#else
    void*   mState;
#endif
};

// ---------------------------------------------------------------------------

#if defined(HAVE_PTHREADS)

inline Condition::Condition() {
    pthread_cond_init(&mCond, NULL);
}
inline Condition::Condition(int type) {
    if (type == SHARED) {
        pthread_condattr_t attr;
        pthread_condattr_init(&attr);
        pthread_condattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
        pthread_cond_init(&mCond, &attr);
        pthread_condattr_destroy(&attr);
    } else {
        pthread_cond_init(&mCond, NULL);
    }
}
inline Condition::~Condition() {
    pthread_cond_destroy(&mCond);
}
inline status_t Condition::wait(Mutex& mutex) {//系统调用wait包含：释放锁、进入休眠等待、唤醒后重新获取锁
    return -pthread_cond_wait(&mCond, &mutex.mMutex);
}
inline status_t Condition::waitRelative(Mutex& mutex, nsecs_t reltime) {//具有时间控制
#if defined(HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE)
    struct timespec ts;
    ts.tv_sec  = reltime/1000000000;
    ts.tv_nsec = reltime%1000000000;
    return -pthread_cond_timedwait_relative_np(&mCond, &mutex.mMutex, &ts);
#else // HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
    struct timespec ts;
#if defined(HAVE_POSIX_CLOCKS)
    clock_gettime(CLOCK_REALTIME, &ts);
#else // HAVE_POSIX_CLOCKS
    // we don't support the clocks here.
    struct timeval t;
    gettimeofday(&t, NULL);
    ts.tv_sec = t.tv_sec;
    ts.tv_nsec= t.tv_usec*1000;
#endif // HAVE_POSIX_CLOCKS
    ts.tv_sec += reltime/1000000000;
    ts.tv_nsec+= reltime%1000000000;
    if (ts.tv_nsec >= 1000000000) {
        ts.tv_nsec -= 1000000000;
        ts.tv_sec  += 1;
    }
    return -pthread_cond_timedwait(&mCond, &mutex.mMutex, &ts);
#endif // HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
}
inline void Condition::signal() {
    pthread_cond_signal(&mCond);
}
inline void Condition::broadcast() {
    pthread_cond_broadcast(&mCond);
}

#endif // HAVE_PTHREADS

// ---------------------------------------------------------------------------
}; // namespace android

　　Barrier是利用上面Condition的一个例子：

class Barrier
{
public:
    inline Barrier() : state(CLOSED) { }//state就是所谓的“条件”
    inline ~Barrier() { }
    void open() {
        Mutex::Autolock _l(lock);
        state = OPENED;
        cv.broadcast();
    }
    void close() {
        Mutex::Autolock _l(lock);
        state = CLOSED;
    }
    void wait() const {
        Mutex::Autolock _l(lock);//临时对象_l，用lock来构造，在AutoLock的构造函数里已给lock加锁（调用lock（）函数）——该wait()函数执行完毕，会自动释放lock(这个场景会使得其他线程再次修改state，产生不安全因素。不过由于Barrier的使用场景的特殊性，其用在线程初始化时，故OK。)
        while (state == CLOSED) {//while语句：不断轮询，直到state==OPENED
            cv.wait(lock);
        }
    }
private:
    enum { OPENED, CLOSED };
    mutable     Mutex       lock;//持有一个互斥锁
    mutable     Condition   cv;//持有一个条件变量
    volatile    int         state;//每次都从内存更新的“条件”
};